1. Field of the Invention
The present invention relates to the use of hybrid copolymers as protective colloids for polymers based on ethylenically unsaturated monomers, to protective colloid-stabilized polymers based on ethylenically unsaturated monomers and to the use thereof, for example in the construction materials industry.
2. Description of the Related Art
Protective colloids find a wide variety of uses for the stabilization of polymers, for example in the dispersion of polymers in water. For instance, protective colloids are used as a stabilizing agent in the preparation of polymers during the polymerization of ethylenically unsaturated monomers. Protective colloids are also used as drying aids in the conversion of polymer dispersions to corresponding water-redispersible powders, also referred to as dispersion powders (DPPs). Various demands are made on protective colloid-stabilized systems. For example, systems of this kind should be storage-stable. A particular aim is to modify dispersion powders such that they remain free-flowing (blocking-stable) and water-redispersible even after storage under compressive or thermal stress. For these purposes, relatively large amounts of protective colloids or inert substances have been added to the polymers to date. For instance, EP-A 632096 describes the spray-drying of aqueous vinyl ester polymers in the presence of polyvinyl alcohol as a spray drying colloid. EP-A 78449 discloses conducting spray-drying with the addition of naphthalenesulfonic acid-formaldehyde condensation products. EP-A 407889 describes the spray-drying of aqueous polymer dispersions in the presence or phenolsulfonic acid-formaldehyde condensation products. EP-A 134451 discloses the production of dispersion powders by spray-drying of aqueous dispersions in the presence of starches or proteins.
However, the addition of standard protective colloids to polymers entails a number of disadvantages. Standard protective colloids are costly and additionally have to be used in relatively large amounts in order to assure sufficient storage stability of the polymer dispersions or dispersion powders, especially redispersibility and blocking resistance. However, a relatively high protective colloid content of the polymer compositions leads inevitably to a lower proportion of the base polymers and hence of the component that actually functions as the active ingredient, for example as a binder. Moreover, high protective colloid contents can especially adversely affect the performance properties of the polymer compositions, for example the rheological properties of corresponding polymer dispersions or polymer redispersions.
In an alternative approach for endowing dispersion powders with blocking-resistant properties, the base polymers are coated with inorganic particles, for example silicate particles (Pickering technology)—as described in US20080220176. The dispersion powders thus obtained have satisfactory blocking stability and water resistance, but inadequate redispersibility in water, since the inorganic particles agglomerate as a result of the drying. The redispersing of agglomerated inorganic particles is technically difficult and at least complex. US20080220176 recommends the use of ultrasound for redispersion. In the standard applications, such as in mineral building materials, corresponding dispersion powders lead to adverse performance properties, the compensation of which in turn necessitates the addition of further specific additives.
Against this background, the problem addressed was that of providing measures for stabilizing polymers, with which the abovementioned disadvantages can be overcome. More particularly, polymers were to be made available in the form of water-redispersible powders (dispersion powders) which are blocking-resistant and additionally have good water redispersibility and, if possible, are to be storage-stable in an environment with relatively high air humidity. Finally, correspondingly stabilized polymers, if possible, were to contain smaller amounts of protective colloids than conventionally stabilized polymers.